In recent years, as the Internet is becoming more widespread, there is an increasing demand for high-speed and high-capacity data communication. In this situation, light transfer systems having excellent high-speed performance have attracted attention and have been vigorously developed. In an optical transmission circuit used in the light transfer system, a drive circuit including a light emitting device, such as a laser diode (LD) or a light emitting diode (LED), is generally employed. However, laser diodes and light emitting diodes have a high level of temperature dependency, i.e., the threshold current increases with an increase in temperature, disadvantageously resulting in a reduction in current-light conversion efficiency. Therefore, when a constant current is invariably output from the drive circuit, optical output power after conversion varies depending on temperature. In particular, the optical output power is disadvantageously low at high temperature. Further, since light is not emitted when a current is lower than or equal to the threshold current, if a bias current (L level of an output current) is lower than the threshold current of a laser diode, the duty factor is also deteriorated.
Therefore, conventionally, in order to invariably output a constant optical output power to obtain stable communication characteristics, the following two methods are used: temperature is measured using a thermistor or the like, and a feedforward control is performed so as to attain a previously set temperature-dependent current value; and the output of a laser diode is monitored using an MPD (monitoring photodiode), and a feedback control is performed so that a current flowing through the MPD is invariably constant. When cost reduction is aimed, the feedforward control method is generally employed in which the cost of the MPD can be eliminated, a feedback-system circuit can be removed, and the chip area can be largely decreased.
According to a conventional technique, a laser diode drive circuit with a temperature compensation circuit comprises a current drive circuit including a differential pair of transistors, a bias circuit for causing a DC bias current to flow through a laser diode, a bias current temperature compensation circuit for controlling a bias current flowing through the bias circuit, depending on ambient temperature, a drive current temperature compensation circuit for controlling a drive current of the laser diode, depending on ambient temperature, and a subtraction circuit, thereby providing a feedforward configuration in which the bias current and the drive current increase with an increase in temperature due to the temperature characteristics of a thermistor and a diode provided therein, so that optical output power can be invariably held constant during transmission (see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-284791